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Investments in Unconsolidated Entities - White Mountain's Investments (Details) | Sep. 30, 2018 | Jan. 24, 2018 | Dec. 31, 2017
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PassportCard |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 50.00% | 50.00% | 50.00%
DavidShield |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 50.00% | 50.00% | 0.00%
Kudu |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 49.50% |  | 0.00%
durchblicker |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 45.00% |  | 45.00%
Compare.com |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 22.10% |  | 22.10%
Tuckerman Capital Fund III, L.P. |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 21.80% |  | 21.30%
White Mountains | PassportCard |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 25.00% |  |
DavidShield | PassportCard |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 25.00% |  |

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